Hardened martensitic steel, method for producing a component from this steel and component obtained in this manner

ABSTRACT

The invention relates to steel which is characterized by the following composition as expressed in percentages by weight:—C=0.18 0.30%, —Co=5-7%, —Cr=2-5%, —Al=1-2%, —Mo+W/2=1-4%, —V=trace 0.3%, —Nb=trace 0.1%, —B=trace−50 ppm, —Ni=10.5-15% with Ni≧7+3.5 Al, —Si=trace 0.4%, —Mn=trace 0.4%, —Ca=trace−500 ppm, —Rare earths=trace−500 ppm, —Ti=trace−500 ppm, —O=trace−200 ppm if the steel is obtained by means of powder metallurgy or trace−50 ppm if the steel is produced in air or under a vacuum from molten metal, —N=trace−100 ppm, —S=trace−50 ppm, —Cu=trace−1%, and —P=trace−200 ppm, the remainder comprising iron and the inevitable impurities resulting from production. The invention also relates to a method of producing a part from said steel and to the part thus obtained.

This is a U.S. National Stage Application of Application No.PCT/FR2006/000877 filed Apr. 20, 2006, claiming priority to FR 0504254,filed Apr. 27, 2005, and FR 0507482, filed Jul. 12, 2005, each of whichis incorporated herein by reference in its entirety.

The invention relates to a martensitic steel which is hardened by meansof a duplex system, that is to say, by means of precipitation ofintermetallic compounds and carbides obtained using appropriatecomposition of the steel and thermal ageing processing operation.

This steel must have:

-   -   a very high level of mechanical strength, but with a high level        of toughness and ductility at the same time, that is to say, a        low level of sensitivity to brittle fracture; this very high        level of strength must remain at high temperatures, that is to        say, at temperatures in the order of 400° C.;    -   good properties in terms of fatigue, which involves in        particular the absence of harmful inclusions such as TiN and        oxides; this feature must be obtained by means of an appropriate        composition and carefully controlled production conditions for        the liquid metal.

Furthermore, it must be case-hardenable and nitridable in order to beable to harden the surface thereof in order to confer thereon a highlevel of resistance to abrasion.

The main application envisaged for this steel is the production oftransmission shafts, in particular for aircraft engines.

The requirement for an excellent level of mechanical strength at hightemperatures does not allow the use, in this application, of carbonsteels whose strength degrades from 200° C. Maraging steels areconventionally used which have a satisfactory level of strength up to350-400° C. Beyond this level, a tempering operation is brought aboutwhich modifies their structure.

A steel composition has been proposed in document U.S. Pat. No.5,393,388 which is intended to improve the heat-resistance and inparticular improve the properties in terms of fatigue, ductility andtoughness. This composition has the disadvantage of requiring a highcontent of Co (from 8 to 16%), which makes the steel very costly.

The object of the invention is to provide a steel which can be used, inparticular for producing mechanical components, such as transmissionshafts or structural elements which have a further improved level ofmechanical strength at high temperatures but also properties in terms offatigue and a level of brittleness which are still suitable for theseapplications. This steel is also intended to have a lower productioncost than the most effective steels currently known for theseapplications.

To this end, the invention relates to a steel, characterised in that thecomposition thereof is, in percentages by weight:

-   -   —C=0.18-0.30%    -   —Co=5-7%    -   —Cr=2-5%    -   —Al=1-2%    -   —Mo+W/2=1-4%    -   —V=trace levels−0.3%    -   —Nb=trace levels−0.1%    -   —B=trace levels−50 ppm    -   —Ni=10.5-15% with Ni≧7+3.5 Al    -   —Si=trace levels−0.4%    -   —Mn=trace levels−0.4%    -   —Ca=trace levels−500 ppm    -   —Rare earth elements=trace levels−500 ppm    -   —Ti=trace levels−500 ppm    -   —O=trace levels−200 ppm if the steel is obtained by means of        powder metallurgy, or trace levels−50 ppm if the steel is        obtained by means of production in air or under vacuum from        liquid metal    -   —N=trace levels−100 ppm    -   —S=trace levels−50 ppm    -   —Cu=trace levels−1%    -   —P=trace levels−200 ppm        the remainder being iron and inevitable impurities resulting        from the production operation.

Preferably, it contains C=0.20-0.25%.

Preferably, it contains Cr=2-4%.

Preferably, it contains Al=1-1.6%, more preferably 1.4-1.6%.

Preferably, it contains Mo≧1%.

Preferably, it contains Mo+W/2=1-2%.

Preferably, it contains V=0.2-0.3%.

Preferably, it contains Nb=trace levels−0.05%.

Preferably, it contains Si=trace levels−0.25%, more preferably tracelevels−0.10%.

Preferably, it contains Mn=trace levels−0.25%, more preferably tracelevels−0.10%.

Preferably, it contains Ti=trace levels−100 ppm.

Preferably, it contains O=trace levels−10 ppm.

Preferably, it contains N=trace levels−50 ppm, more preferably tracelevels−10 ppm.

Preferably, it contains S=trace levels−10 ppm, more preferably tracelevels−5 ppm.

Preferably, it contains P=trace levels−100 ppm.

Preferably, the martensitic conversion temperature Ms thereof is greaterthan or equal to 140° C., with Ms=550−350×C %−40×Mn %−17×Cr %−10×Mo%−17×Ni %−8×W %−35×V %−10×Cu %−10×Co %+30×Al %° C.

The invention also relates to a method for producing a component fromsteel, characterised in that it comprises the following steps prior tothe finishing of the component which provides the component with itsdefinitive shape:

-   -   the preparation of a steel having the above composition;    -   the forging of this steel;    -   a softening tempering operation at 600-675° C. for from 4 to 20        hours followed by cooling in air;    -   solution heat treatment at 900-1000° C. for at least 1 hour,        followed by cooling in oil or air which is sufficiently rapid to        prevent the precipitation of intergranular carbides in the        austenite matrix;    -   optionally a cryogenic processing operation at −50° C. or lower,        preferably at −80° C. or lower, in order to convert all the        austenite into martensite, the temperature being 200° C. or more        lower than Ms, at least one of the processing operations lasting        at least 2 hours;    -   optionally a processing operation for softening the martensite        so obtained carried out at 150° C.-250° C. for from 4-16 hours,        followed by cooling in still air;    -   a hardening ageing operation at 475-600° C., preferably from        490-525° C. for from 5-20 h.

The component may also be subjected to a nitriding operation or acase-hardening operation.

The invention also relates to a mechanical component or structuralelement component, characterised in that it is produced in accordancewith the above method.

It may be inter alia an engine transmission shaft, an engine suspensiondevice or a landing gear element.

As will have been appreciated, the invention is firstly based on a steelcomposition which is distinguished from the prior art in particular by alower content in terms of Co. The contents of the other alloy elementsare adjusted in accordance therewith, in particular the contents of Al,Mo, W, Ni. An optimised thermal processing operation is also proposed.

These steels have a plastic domain (difference between tensile strengthR_(m) and yield strength R_(p0.2)) which is midway between those ofcarbon steels and maraging steels. For maraging steels, the differenceis very low, resulting in a high yield strength but a rapid rupture assoon as it is exceeded. The steels of the invention have, in thisrespect, properties which can be adjusted by the proportion of hardeningphases and/or carbon.

The steel of the invention may be machined in the quenched state, usingtools which are suitable for a hardness of 45 HRC. It is midway betweenmaraging steels (which can be machined in the unprocessed quenched statesince they have a soft martensite with low carbon) and carbon steelswhich must be machined in the annealed state.

The invention is based on obtaining a hardening operation which iscarried out conjointly using intermetallic compounds of the type β-NiAland carbides of the type M₂C, and on the presence of reversion austeniteformed during the hardening ageing operation, which provides ductilityfor the martensite by forming a sandwich structure (a few % of reversionaustenite between the laths of martensite).

The formation of nitrides must be prevented, in particular Ti and Alwhich have an embrittling effect; any addition of Ti is thereforeeliminated (maximum allowed: 500 ppm, more preferably 100 ppm), and N islimited to a value as low as possible, by fixing it in order to preventthe formation of AlN.

The M₂C carbides of Cr, Mo, W and V which contain very little Fe arepreferred for their hardening and non-embrittling properties. They arestabilised by Mo and W. The total of the content in terms of Mo and halfof the content of W must be at least 1%. Mo+W/2=4% must not be exceededso as not to impair the forgeability and not to form intermetalliccompounds of the μ phase of the type Fe₇Mo₆ (see also Cr and V).Preferably, Mo+W/2 is between 1 and 2%.

Cr and V are precursors to the stabilisation of the M₂C which are“metastable” carbides. V forms carbides which “block” the joints of thegrains and limit the enlargement of the grains during thermal processingoperations at high temperature. V=0.3% must not be exceeded so as not topromote the formation of undesirable intermetallic compounds of the μphase. Preferably, the content of V is between 0.2 and 0.3%.

The presence of Cr (at least 2%) allows the level of V carbides to bedecreased and the level of M₂C to be increased. 5% must not be exceededso as not to form μ phase, then M₂₃C₆ carbides. Preferably, 4% is notexceeded.

The presence of C promotes the appearance of M₂C relative to the μphase. However, an excessive content causes segregations and a reductionof Ms. The content thereof must be between 0.18 and 0.30%, preferablyfrom 0.20-0.25%.

Co delays the restoration of the dislocations and therefore slows downthe excessive ageing mechanisms in the martensite at high temperatures.It thus allows a high level of heat resistance to be maintained.However, it is suspected that, since Co promotes the formation of the μphase mentioned above, which is the one which hardens the maragingsteels of the prior art having Fe—Ni—Co—Mo, the significant presencethereof contributes to reducing the quantity of Mo and/or W available toform M₂C carbides which contribute to the hardening according to themechanism which it is desirable to promote. The content of Co proposed(from 5 to 7%) in combination with the contents of other elements, isthe result of a compromise between these various advantages anddisadvantages.

Ni and Al are connected. If the level of Al is too high relative to Ni,there is no longer any potential for reversion austenite. If there istoo much Ni, the level of hardening phase of the type NiAl is reduced toan excessive extent and Al remains largely in solution. At the end ofthe quenching operation, there must be no residual austenite and amartensitic structure must be left. To this end, if a quenchingoperation with solid CO₂ is used, Ms must be ≧140° C. Ms is calculatedin accordance with the conventional formula: Ms=550−350×C %−40×Mn%−17×Cr %−10×Mo %−17×Ni %−8×W %−35×V %−10×Cu %−10×Co %+30×Al %° C. Thecontent of Ni must be adjusted to this end in accordance with the otherelements. It is necessary to have Al=1-2%, preferably from 1-1.6%, morepreferably from 1.4-1.6% and Ni=10.5-15%, with Ni≧7+3.5 Al. Ideally,there is 1.5% of Al and from 12-13% of Ni. These conditions promote thepresence of NiAl which increases the tensile strength R_(m) which hasalso been found not to deteriorate with a relatively low content of Co.The yield strength R_(p0.2) is influenced in the same manner as R_(m).

Compared with the steels known from U.S. Pat. No. 5,393,388, in which avery high presence of reversion austenite is desired in order to providea high level of ductility and toughness, the invention promotes thepresence of the hardening B2 phases, in particular NiAl, in order toobtain a high level of mechanical strength at high temperatures.Compliance with the conditions set out with respect to Ni and Al ensuresa potential content of reversion austenite sufficient to retain a levelof ductility and toughness appropriate for the applications envisaged.

It is possible to add B, but no more than 50 ppm so as not to degradethe forgeability of the steel.

A feature of the invention is also the possibility of replacing at leasta portion of Mo with W. With an equivalent atomic fraction, W segregatesless during solidification than Mo and provides mechanical strength athigh temperatures by the formation of carbides which are very stable interms of temperature. It has the disadvantage of being costly and it ispossible to optimise the cost by associating it with Mo. As stated,Mo+W/2 must be between 1 and 4%, preferably between 1 and 2%. It ispreferable to retain a minimum content in terms of Mo of 1% in order tolimit the cost of the steel.

N may be up to 100 ppm if the steelmaking is carried out in air and if Nis fixed in carbonitrides of Nb and/or V in order to prevent theformation of the embrittling nitride AlN. It is preferable to carry outthe steelmaking under vacuum in order to have N≦50 ppm, or ≦10 ppm.

Cu may be up to 1%. It is capable of being involved in the hardeningoperation using its epsilon phase and the presence of Ni allows theharmful effects thereof to be limited.

Generally, elements which are able to segregate at the joints of thegrains and embrittle them, such as P and S, must be controlled withinthe following limits: S=trace levels−50 ppm, preferably trace levels−10ppm, more preferably trace levels−5 ppm, and P=trace levels−200 ppm,preferably trace levels−100 ppm.

It is possible to use Ca as a deoxidising agent, it being ultimatelyfound in residual quantities (≦500 ppm). In the same manner, residues ofrare earth elements may ultimately remain (≦500 ppm) following aprocessing operation for refining the liquid metal.

The acceptable content of oxygen varies depending on whether the steelhas been obtained by means of powder metallurgy or by means ofproduction from liquid metal in air or under vacuum. In the firstinstance, a content of up to 200 ppm is tolerated. In the secondinstance, the maximum content is 50 ppm, preferably 10 ppm.

By way of examples, samples of steel were tested whose compositions (inpercentages by weight) are set out in Table 1:

TABLE 1 Composition of the samples tested A B C D E (reference)(reference) (invention) (invention) (invention) C % 0.233 0.247 0.2390.244 0.247 Si % 0.082 0.031 0.031 0.037 0.030 Mn 0.026 0.030 0.0330.033 0.030 % S 1.0 7.3 3.8 6.1 6.7 ppm P 54 <30 <30 <30 <30 ppm Ni %13.43 13.31 12.67 12.71 13.08 Cr % 2.76 3.08 3.38 3.38 3.29 Mo 1.44 1.531.52 1.53 1.53 % Al % 0.962 1.01 1.50 1.50 1.49 Co % 10.25 10.35 6.186.24 6.33 Cu % 0.014 <0.010 0.011 0.012 0.011 Ti % <0.020 <0.020 <0.020<0.020 <0.020 Nb % <0.0050 <0.0050 <0.0050 <0.0050 0.054 B <10 <5 <5 29<5 ppm Ca <50 <50 <50 <50 <50 ppm N <3 13 13 12 14 ppm O <3 4.8 3.4 4.47.7 ppm V % <0.010 0.252 0.245 0.254 0.253

Reference steel A corresponds to a steel according to U.S. Pat. No.5,393,388, therefore having a high content of Co.

Reference steel B corresponds to a steel which is comparable to steel A,to which V has been added without the content of Co being modified.

Steel C corresponds to the invention, in particular in that, comparedwith steels A and B, the Al content thereof has been increased and theCo content thereof has been decreased.

Steel D according to the invention was further subjected to an additionof B.

Steel E according to the invention was further subjected to an additionof Nb.

These samples were forged from ingots of 200 kg in the form of flat barsof 75×35 mm under the following conditions. A homogenisation processingoperation of at least 16 hours at 1250° C. is followed by a firstforging operation which is intended to divide the rough structures ofthe ingots; semi-finished products having a cross-section of 75×75 mmwere then forged after being brought to temperature again at 1180° C.;finally, each semi-finished product was placed in an oven at 950° C.,then forged at this temperature in the form of flat bars of 75×35 mmwhose granular structure is refined by these successive operations.

After forging, the samples were subjected to:

-   -   solution heat treatment at 900° C. for 1 hour, then cooling in        air;    -   a cryogenic processing operation at −80° C. for 8 hours;    -   a hardening ageing operation at 495° C. for 5 hours, then        cooling in air.

The properties of the samples (tensile strength R_(m), elastic limitRp0.2, elongation A5d, contraction Z, resilience KV, HRC hardness, ASTMgrain size) are set out in Table 2. In this instance, they are measuredat normal ambient temperature.

TABLE 2 Properties of the samples tested R_(m) Rp0.2 A5d Z KV Grain(Mpa) (Mpa) (%) (%) (J) HRC ASTM A 2176 1956 11.2 58 25/27 55.3 8 B 22182002 9.9 56 26/30 56.3 8/9 C 2316 2135 9.5 49 20/24 57.6 8 D 2328 19978.9 43 21/22 57.9 8 E 2303 1959 10 47 16/19 57.6 9

It can be seen that the samples C, D and E according to the inventionhave a tensile strength which is far greater than that of the referencesamples A and B. The elastic limit is at least of the same order ofmagnitude. In contrast to this measurement of the tensile strength, theproperties of ductility (reduction of area and elongation at break) andresilience are reduced if the thermal processing operation described iscarried out.

The reference sample B indicates that the addition of only V to thesteel A brings about an improvement of only specific properties, and inproportions which are most often less than in the case of the invention.

In particular, the increase of Al in the case of the invention, togetherwith retaining a high content of Ni, renders the hardening phase NiAlmore present and is a substantial factor in the improvement of thetensile strength.

The additions of B and Nb of the samples D and E respectively are notnecessary in order to obtain the high levels of mechanical strengthwhich are the intended priority in the invention.

Additional experiments which were carried out, in particular on sampleC, have allowed it to be determined that, in addition to the processingoperations carried out, a softening tempering operation at a temperatureof at least 600° C. prior to the dissolution was necessary in order toobtain a complete recrystallisation of the steel during the solutionheat treatment. This softening tempering operation may, for example, becarried out at 650° C. for 8 hours and may be followed by cooling inair. Owing to this, the products directly resulting from thethermomechanical transformation may be readily subjected to thefinishing operations (rectification, peeling, machining . . . ) whichconfer on the component its definitive shape.

After this softening tempering operation at 650° C. for 8 hours andcooling in air, a solution heat treatment at 935° C. for one hour,followed by cooling in oil, then a cryogenic processing operation at−80° C. for 8 hours, then a stress-relieving operation at 200° C. for 8hours (on tensile test-pieces) or 16 hours (on resilience test-pieces),then an ageing operation at 500° C. for 12 hours, followed by cooling inair allowed an ASTM grain size of 8 and the following mechanicalproperties to be obtained:

-   -   in a longitudinal direction, at 20° C.: R_(m)=2271 MPa;        R_(p0.2)=1983 MPa; A5d=11.8%; Z=57%; KV=27 J;    -   in a transverse direction at 20° C.: R_(m)=2273 MPa;        R_(p0.2)=2023 MPa; A5d=8.8%; Z=41%; KV=22−24 J;    -   in a longitudinal direction at 400° C.: R_(m)=1833 MPa;        R_(p0.2)=1643 MPa; A5d=11.1%; Z=58%.

In a longitudinal direction at 20° C., there is therefore an excellentcompromise between tensile strength, ductility and resilience. In thetransverse direction, the resilience values remain acceptable. And, at400° C., the tensile strength remains very high and the steel of theinvention therefore overcomes the problems addressed in a very effectivemanner.

Generally, an optimised method for thermally processing the steelaccording to the invention in order to ultimately obtain a componentwhich has the desired properties is, after forging the blank of thecomponent and before the finishing operation which gives the componentits definitive shape:

-   -   softening annealing operation at 600-675° C. for from 4 to 20        hours followed by cooling in air;    -   solution heat treatment at 900-1000° C. for at least one hour,        followed by cooling in oil or air which is sufficiently rapid to        prevent the precipitation of intergranular carbides in the        matrix of austenite;    -   optionally a cryogenic processing operation at −50° C. or lower,        preferably at −80° C. or lower, in order to convert all the        austenite into martensite, the temperature being 200° C. or more        lower than Ms, at least one of the processing operations lasting        at least 2 hours; for the compositions which have in particular        a relatively low content of Ni, this cryogenic processing        operation is less advantageous;    -   optionally a processing operation for softening the martensite        obtained after quenching carried out at 150-250° C. for from        4-16 hours, followed by a cooling operation in still air;    -   a hardening ageing operation at 475-600° C., preferably from        490-525° C. for from 5 to 20 hours.

The preferred applications of the steel according to the invention aredurable components for mechanical engineering and structural elements,for which it is necessary to have a tensile strength at low temperaturesof between 2200 MPa and 2350 MPa, combined with values of ductility andresilience which are at least equivalent to those of the besthigh-strength steels, and at high temperatures (400° C.) a tensilestrength in the order of 1800 MPa and optimum fatigue properties.

The steel according to the invention also has the advantage of beingcase-hardenable and nitridable. It is therefore possible to confer ahigh level of abrasion resistance on the components which use thissteel. This is particularly advantageous in the cited envisagedapplications.

The invention claimed is:
 1. Steel, characterised in that thecomposition thereof is, in percentages by weight: —C=0.18-0.30% —Co=5-7%—Cr=2-5% —Al=1-2% —Mo+W/2=1-4% —V=trace levels−0.3% —Nb=tracelevels−0.1% —B=trace levels−50 ppm —Ni=10.5-15% with Ni≧7+3.5 Al—Si=trace levels−0.4% —Mn=trace levels−0.4% —Ca=trace levels−500 ppm—Rare earth elements=trace levels−500 ppm —Ti=trace levels−500 ppm—O=trace levels−200 ppm if the steel is obtained by means of powdermetallurgy, or trace levels−50 ppm if the steel is obtained by means ofproduction in air or under vacuum from liquid metal —N=trace levels−100ppm —S=trace levels−50 ppm —Cu=trace levels−1% —P=trace levels−200 ppmthe remainder being iron and inevitable impurities resulting from theproduction operation, wherein Rm at ambient temperature is between 2200and 2350 MPa.
 2. Steel according to claim 1, characterised in that itcontains C=0.20-0.25%.
 3. Steel according to claim 1, characterised inthat it contains Cr=2-4%.
 4. Steel according to claim 1, characterisedin that it contains Al=1-1.6%.
 5. Steel according to claim 1,characterised in that it contains Mo=1-4%.
 6. Steel according to claim1, characterised in that it contains Mo+W/2=1-2%.
 7. Steel according toclaim 1, characterised in that it contains V=0.2-0.3%.
 8. Steelaccording to claim 1, characterised in that it contains Nb=tracelevels−0.05%.
 9. Steel according to claim 1, characterised in that itcontains Si=trace levels−0.25%.
 10. Steel according to claim 1,characterised in that it contains Mn=trace levels−0.25%.
 11. Steelaccording to claim 1, characterised in that it contains Ti=tracelevels−100 ppm.
 12. Steel according to claim 1, characterised in that itcontains 0=trace levels−10 ppm.
 13. Steel according to claim 1,characterised in that it contains N=trace levels−50 ppm.
 14. Steelaccording to claim 1, characterised in that it contains S=tracelevels−10 ppm.
 15. Steel according to claim 1, characterised in that itcontains P=trace levels−100 ppm.
 16. Steel according to claim 1,characterised in that the martensitic conversion temperature Ms thereofis greater than or equal to 140° C., withMs=550−350×C%−40×Mn%−17×Cr%−10×Mo%−17×Ni%−8×W%−35×V%−10×Cu%−10×Co%+30×Al%°C.
 17. Steel according to claim 1, characterised in that it containsAl=1.4-1.6%.
 18. Steel according to claim 1, characterised in that itcontains Si=trace levels−0.10%.
 19. Steel according to claim 1,characterised in that it contains Mn=trace levels−0.10%.
 20. Steelaccording to claim 1, characterised in that it contains N=tracelevels−10 ppm.
 21. Steel according to claim 1, characterised in that itcontains S=trace levels−5 ppm.
 22. Steel according to claim 1,characterised in that it contains Cr=3.29-5%.
 23. Steel according toclaim 1, wherein Rm is about 1800 MPa at 400° C.